The objective of the proposed study are: (1) to determine the role of central nervous system actions of vasopressin (AVP) in the reflex hemodynamic and neural effects of alterations in vagal afferent activity; (2) to assess the role of the area postrema in mediating central nervous system effects of AVP on the cardiopulmonary baroreflex; and (3) to determine if AVP interacts centrally with the cardiopulmonary baroreflex to alter reflex effects on renal sympathetic nerve activity. Hemodynamic effects of activation or blockade of central AVP actions on the cardiopulmonary baroreflex will be evaluated by intravertebral administration of AVP or a specific AVP antogonist during changes in vagal afferent activity in the conscious dog. The importance of the area postrema in mediating this central interaction will be assessed by comparing responses in normal conscious dogs and in dogs subjected to area postrema ablation. A central interaction of AVP with the sympathetic nervous system during alterations in cardiopulmonary baroreflex activity will be evaluated by direct recording of efferent renal sympathetic nerve activity in the conscious rabbit.